Abstract: This paper examines the influence of different control methods of the year-end water stage in a multi-year regulating reservoir and the inter-annual variations in its inflow on the resulted operation of the cascade system, through a case study of the cascade reservoirs in the middle and lower Yalong River. We conduct optimization calculations of multi-dimensional dynamic programming and compare three operating methods - fixed stages, stage variations by a specified function, and stage variations under dynamic bounds - for controlling the year-end stage in the Lianghekou Reservoir, and work out a new optimal control method that can achieve coupling optimization with inflow frequency variations. This study is useful to improve the joint operation and the social and economic benefits of the cascade reservoirs with multi-year regulating reservoirs.

Key words: multi-year regulating reservoir, year-end water stage, control mode, dynamic control, Yalong River, Lianghekou Reservoir